A commonly used method of measuring the power level of a high frequency signal, or of measuring the amplitude response characteristics of a circuit by applying a sweep frequency signal thereto and measuring the amplitude variations of the resultant output signal, involves the use of a device such as a barretter as a sensor for the high frequency signal being measured. Such a device displays a change in resistance which is proportional to the amplitude of a high frequency signal which is applied to it, and thus can be used for example in a bridge circuit in conjunction with capacitors and resistors to produce an output signal which indicates the amplitude or power level of a high frequency signal coupled to the sensor. Use of a device such as a barretter for this purpose brings a number of disadvantages. First, since it is normally necessary to measure power or amplitude levels down to very low levels, it is usual to utilize an amplifier circuit which is of the chopper type, incorporating a DC to AC converter circuit. This DC to AC conversion is normally performed at a fixed frequency, and produces noise spikes in the resultant output signal. When the amplitude of the signal to be measured is very low, this spike noise makes it difficult to observe the amplified signal being measures, when this is for example being displayed on a cathode ray tube display. If it is attempted to remove this spike noise by increasing the frequency of the DC to AC conversion process and by filtering out this noise, the result is to introduce an effective drift voltage into the measured signal, leading to reduced accuracy of measurement. Another disadvantage of such a conventional system is that, due to the low response speed of a sensor such as a barretter, it is difficult or impossible to utilize the output signal to observe an amplitude response characteristic on a cathode ray tube display.
It is also possible to measure the power or amplitude level of a high frequency signal using a device such as a diode as a sensor. Such a sensor has a very high speed of response, and has a linear response to changes in input signal power level at low signal levels, but displays increasing non-linearity as the amplitude of the input signal is increased above a certain level.
With the present invention, the disadvantages of such a diode type of power level sensor are eliminated, by applying linearity compensation to the output signal from the power level sensor when the input signal level exceeds a predetermined reference level.